1. Field of the Invention
The present invention pertains to telecommunications, and particularly to interworking of differing transport technologies in a telecommunications system.
2. Related Art and other Considerations
In a typical cellular radio system, mobile user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified, typically by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are connected to a core network.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UTRAN is a third generation system which is in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a wideband code division multiple access (W-CDMA) system.
As those skilled in the art appreciate, in W-CDMA technology a common frequency band allows simultaneous communication between a user equipment unit (UE) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed code, such as a pseudo-noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment units (UEs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. The high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal. In CDMA, therefore, a user equipment unit (UE) need not switch frequency when handoff of a connection is made from one cell to another. As a result, a destination cell can support a connection to a user equipment unit (UE) at the same time the origination cell continues to service the connection. Since the user equipment unit (UE) is always communicating through at least one cell during handover, there is no disruption to the call. Hence, the term “soft handover.” In contrast to hard handover, soft handover is a “make-before-break” switching operation.
The Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN) accommodates both circuit switched and packet switched connections. In this regard, in UTRAN the circuit switched connections involve a radio network controller (RNC) communicating with a mobile switching center (MSC), which in turn is connected to a connection-oriented, external core network, which may be (for example) the Public Switched Telephone Network (PSTN) and/or the Integrated Services Digital Network (ISDN). On the other hand, in UTRAN the packet switched connections involve the radio network controller communicating with a Serving GPRS Support Node (SGSN) which in turn is connected through a backbone network and a Gateway GPRS support node (GGSN) to packet-switched networks (e.g., the Internet, X.25 external networks).
There are several interfaces of interest in the UTRAN. The interface between the radio network controllers (RNCs) and the core network(s) is termed the “Iu” interface. The interface between a radio network controller (RNC) and its base stations (BSs) is termed the “Iub” interface. The interface between the user equipment unit (UE) and the base stations is known as the “air interface” or the “radio interface” or “Uu interface”. In some instances, a connection involves both a Serving or Source RNC (SRNC) and a target or drift RNC (DRNC), with the SRNC controlling the connection but with one or more diversity legs of the connection being handling by the DRNC (see, in this regard, U.S. patent application Ser. No. 09/035,821 filed Mar. 6, 1998, entitled “Telecommunications Inter-Exchange Measurement Transfer”; and U.S patent application Ser. No. 09/035,788 filed Mar. 6, 1998, entitled “Telecommunications Inter-Exchange Congestion Control”, both of which are incorporated herein by reference). The interface between a SRNC and a DRNC is termed the “Iur” interface.
In the Universal Mobile Telecommunications (UMTS), a service is identified on a non-access stratum level of the UMTS architecture by a Non-Assess Stratum (NAS) Service Identifier (NAS Service ID). On the access stratum level of the UMTS architecture, each service is identified by a radio access bearer (RAB) identifier (RAB ID) on the Iu interface and by one or more radio bearer (RB) identifiers (RB IDs) on the radio interface (e.g., the air interface). Each NAS Service is thus linked to one radio access bearer (RAB), and each radio access bearer (RAB) is linked to one or more radio bearers (RBs). One or more radio bearers (RBs) are linked to one transport channel, e.g., to a common transport channel or to a Dedicated Transport Channel (DCH), on the Iur, Iub, and radio interfaces.
A project known as the Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM-based radio access network technologies. In a radio access network such as UTRAN as specified by 3GPP R99 standards, there is a need to transport data between nodes of the radio access network. These nodes could be radio network controller (RNC) nodes and base station nodes, for example. For example, the transport of data can be between two radio network controller (RNC) nodes, or between a radio network controller (RNC) node and a base station node.
In general there are two basic categories of data that are transported between nodes of the radio access network. A first type of data is user data, which is generally data that is eventually carried over the radio (air) interface (data that is transmitted to or received from the user equipment unit (UE) over the air interface). A second type of data is control signaling, e.g., signaling for control between the nodes within the radio access network. Normally, the transport of these two types of data is organized as separate transport networks. That is, user data is carried on a user data transport network, while the control signaling is carried on a signaling transport network. There is also control data associated with the user data, such control data being carried on the user data transport network.
For the UMTS R99 standard as specified by the Third Generation Partnership Project (3GPP), AAL2/ATM was selected as the user data transport in the WCDMA radio access network (e.g., the UTRAN). Asynchronous Transfer Mode (ATM) technology (ATM) is a packet-oriented transfer mode which uses asynchronous time division multiplexing techniques. Packets are called cells and have a fixed size. An ATM cell consists of 53 octets, five of which form a header and forty eight of which constitute a “payload” or information portion of the cell. The header of the ATM cell includes two quantities which are used to identify a connection in an ATM network over which the cell is to travel, particularly the VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier). In general, the virtual path is a principal path defined between two switching nodes of the network; the virtual channel is one specific connection on the respective principal path.
A protocol reference model has been developed for illustrating layering of ATM. The protocol reference model layers include (from lower to higher layers) a physical layer (including both a physical medium sublayer and a transmission convergence sublayer), an ATM layer, and an ATM adaptation layer (AAL), and higher layers. The basic purpose of the AAL layer is to isolate the higher layers from specific characteristics of the ATM layer by mapping the higher-layer protocol data units (PDU) into the information field of the ATM cell and vise versa.
There are several differing AAL types or categories, including AAL0, AAL1, AAL2, AAL3/4, and AAL5. AAL2 is a standard defined by ITU recommendation I.363.2. An AAL2 packet comprises a three octet packet header, as well as a packet payload. The AAL2 packet header includes an eight bit channel identifier (CID), a six bit length indicator (LI), a five bit User-to-User indicator (UUI), and five bits of header error control (HEC). The AAL2 packet payload, which carries user data, can vary from one to forty-five octets.
Although AAL2/ATM was selected as the user data transport in the WCDMA radio access network (e.g., the UTRAN) for the 3GPP R99 standard, the UTRAN architecture is structured to accommodate transport technologies other than ATM. An aspect of the UTRAN which facilitates such accommodation of other transport technologies is the fact that the UTRAN is carefully layered into a radio network layer and a transport layer. In an example shown in FIG. 1, the radio network layer is above line L, while the transport layer is below line L. The radio network layer and the transport layer each have a control plane and a user plane. For the transport of user data, this layering implies that separate frame handling (FP) protocols have been specified as part of the radio network layer (e.g., for the formatting of user data and appending of associated control data). This frame handling (FP) protocol assumes certain services from the transport layer.
FIG. 1 illustrates a first node 1-26 which operates in accordance with the 3GPP R99 standards. Node 1-N is another node which also operates in accordance with the 3GPP R99 standards. The node 1-26 and node 1-N communicate across an Iux interface. In the descriptor “Iux interface”, the “x” indicates that the interface is generic and can be, for example, the Iub interface (in which case node 1-N is a base station node), or the Iur interface (in which case node 1-N is another RNC node), or the Iu interface (in which case node 1-N is a core network node). As mentioned above, FIG. 1 does not show the physical layer, which can be a link, preferably bidirectional, between nodes 1-26 and 1-N.
FIG. 1 further shows several types of signaling between node 1-26 and node 1-N. In the control plane of the radio network layer, both node 1-26 and node 1-N execute an “application” which involves application control signaling between node 1-26 and node 1-N. In FIG. 1, the application control signaling is depicted by the line labeled as “A-XP”. In the user plane of the radio network layer, user plane traffic occurs as indicated by the line labeled “Iux FP”.
In accordance with the 3GPP R99 standard, the establishment of a transport bearer is usually (but not always) initiated by the serving radio network controller (SRNC) as part of the execution of a radio network layer (RNL) procedure. The radio network layer procedure by which the SRNC initiates the transport bearer has four basic steps. As a first step, the node which initiates the (RNL) procedure sends an application message (the application initiating control message) in the control plane of the radio network layer to the other node, initiating the radio network layer procedure. For the Iu interface, the application initiating control message includes an address and a reference (e.g., a binding identification). As a second step, the node receiving the application initiating application message returns an application initiation response message. For establishment of a transport bearer over the Iur and Iub interfaces, the application initiation response message includes an address and a reference (e.g., binding identification) for the receiving node (e.g., UMTS node 1-N in FIG. 1). As a third step, the SRNC sends a transport bearer establishment request message using transport-specific signaling within the transport layer. The Access Link Connection Application Protocol (ALCAP) has been employed by 3GPP as a generic name to indicate the protocol for establishment of transport bearers, e.g., a name for transport layer control plane signaling. For the AAL2/ATM user data transport scheme implemented by 3GPP R99 standards, the ALCAP protocol is Q.2630.1 protocol, also known as q.aal2. The receiving node (e.g., UMTS node 1-N in FIG. 1) receives the transport bearer related information carried in the transport bearer establishment request message and associates the transport bearer with the previous radio network layer procedure using the address and reference (e.g., the address and reference returned in the second above-described message, for example, in the case of the Iur or Iub interfaces). As a fourth step, the receiving node sends back an establish confirm message. The transport bearer is not established until the initiating node receives the establish confirm message. When the transport bearer is established, it is communicated to the higher layers of the UMTS node initiating the RNL procedure (e.g., UMTS node 1-26).
In view of the UTRAN architecture being structured to accommodate transport technologies other than ATM, internet protocol (IP) has been considered as another transport technology. In this regard, some adaptation protocol on top of IP may be required in order to provide services to the frame handling (FP) protocols as are provided by the AAL2 protocol. One example of such an adaptation protocol is the XTP protocol which is disclosed in U.S. patent application Ser. No. 09/734,040, filed Dec. 12, 2000, which is incorporated herein by reference. Such adaptation protocol provides, among other things, both connection identification in endpoints of the IP network and in-sequence delivery. There are two main ways of establishing an IP transport bearer. One way is to include the IP address and IP endpoint identifier in the radio network layer application signaling, and to exchange the IP address and IP endpoint identifier in both directions. The other way is to have separate signaling to exchange this information.
Interworking between AAL2 and internet protocol (IP) has heretofore involved both the control plane of the radio network layer and the transport layer. In this case the application layer signaling is performed in two stages. The first stage involves signaling first to a node that serves as a gateway for the signaling. At the gateway node the application signaling is terminated, and the transport-related information is changed from ATM to IP and vice versa. This transport information is then provided to the transport layer of the transport interworking function. However, for some applications such as the 3GPP UTRAN, this state of the art interworking does not meet the architectural requirement to separate the transport technology from the radio network layer (e.g., application) signaling.
When introducing a new transport technology such as internet protocol (IP), migration aspects of already-implemented networks must be considered. This means, for example, that it must be possible for an operator to operate the network where parts of UTRAN use AAL2/ATM transport, and other parts use internet protocol (IP). Moreover, the new nodes must be able to interwork in a backward compatible way with the old nodes. Given the consideration that all RNC nodes within a radio network such as UTRAN should be able to reach each other, introduction of the new transport technology can be problematic. One possible solution could be to require that all new nodes which connect to the internet protocol (IP) network also connect to the existing AAL2/ATM network to allow reachability over both transport networks. However, such a requirement puts unrealistic constraints on deployment of new nodes.
What is needed, and an object of the present invention, is a technique for interworking differing transport technologies in a multi-layer telecommunications system which includes an application layer and a transport layer.